A 20 long is fine for an adult Kenyan. They reach breeding size in 3-4 years, but some breeders push them at 2 years of age (with some trade-offs). http://www.kingsnake.com/sandboa/breeding.html

Jason Hess (who I got my 3 sand boas, rosy and rack from) has some Anery babies for sell and will soon be adding some het Anery babies on his available page. He may also have some adults he's willing to part with (not listed on the site). Just email him (tell him I sent ya). He is great to do business with, very professional, takes great care of his animals (I've seen his herp room) and gives you a written log of each snakes information (eating, shedding, weight, etc).
http://www.jasons-jungle.com/index.html